Post on 23-Dec-2015
Photoelectron Spectroscopy
• Lecture 5 – instrumental details– General spectrometer design– Vacuum generation and measurement
PhotonSource
Sample Electron KineticEnergy Analyzer
ElectronDetector
Electron Amplifier,Counter, and Recorder
VacuumPumps
Required spectrometer components
Spectrometer design considerations
• What type of samples are of interest?• What resolution is required to gather the information of
interest?– Resolution of this experiment is always instrumental based.– Experimental resolution is directly correlated with electron kinetic
energy.
• What can be done to increase experimental sensitivity?– Often a pay-off between resolution and sensitivity.
• What is it going to cost?– Money for constructing spectrometer– Pumping requirements– Time for data collection
Gas-Phase Photoelectron Spectroscopy• Atoms, neutral Molecules, anions, clusters, etc.• For neutral molecules, need a vapor pressure of ≈10-4 torr in high vacuum at temperatures <≈ 500 °C• “Hot” molecules will have more complicated vibrational/rotational contributions.
Condensed-Phase Photoelectron Spectroscopy• Film on conductive surface• For valence spectroscopy, need uniform film
•(vapor deposition, SAMs, spin coating)
Sample Considerations
• Low pressure is required for operation of electron detectors
• Pressure must be low enough to allow mean-free-path of electrons through the analyzer
• Pressure must be low enough that gas-phase samples are volatile
• Ultra-high vacuum is required to lower surface contamination for condensed-phase spectroscopy
• Vacuum pump: a pump that removes gas molecules from a sealed volume in order to leave behind a partial vacuum
Why do we need vacuum?
Atmospheric pressure 760 torr
Low vacuum 760 to 25 torr
Medium vacuum 25 to 10-3 torr
High vacuum 10-3 to 10-9 torr
Ultra high vacuum 10-9 to 10-12 torr
Extremely high vacuum <10-12 torr
Vacuum Ranges
Vacuum cleaner 600 torr
Liquid ring vacuum pump 24 torr
Freeze drying 1 to 0.1 torr
Rotary vane pump 1 to 10-3 torr
Incandescent light bulb 0.1 to 0.01 torr
Thermos bottle 10-2 to 10-3 torr
Near earth outer space 10-6 torr
Turbopumped vacuum chamber 10-6 to 10-9 torr
Cryopumped surface science chamber 10-9 to 10-11 torr
Pressure on the moon 10-11 torr
Interstellar space 10-17 torr
Examples of vacuum levels
How Low Must Pressure be for a Surface to be “Clean”?
If “sticking coefficient” S = 1And pressure = 2.5 x 10-6 TorrA monolayer will form in 1 second
Lower pressure to ~ 10-9 TorrA monolayer forms in 1,000 secondsS is usually <<1
Methods for Vacuum Generation: 1
• Positive displacement: use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere (rotary vane, scroll pump, roots blower)
Methods for Vacuum Generation: 2• Momentum transfer: use high speed jets of fluid or
rotating blades to knock gaseous molecules out of the chamber (diffusion, turbomolecular)
Methods for Vacuum Generation: 3
• Entrapment: capture gases in a solid or absorbed state (cryopumps, getters, ion pumps)
Vacuum measurement
McLeod gauge> 10-4 torr
Bourdon gauge> 10-2 torr
thermocouple gauge760 - 10-3 torr
ion gauge10-3 - 10-10 torr